1. Field of the Invention
The present invention relates to an improved and substantially simplified method of producing dihydromyrcenol, using dihydromyrcenyl chloride as the starting material.
2. Description of the Background
Dihydromyrcenol is a monoterpene which has many uses in the fragrance industry. Derivatives of dihydromyrcenol such as ethers and esters are particularly important.
Dihydromyrcenyl chloride is converted directly into the alcohol by hydrolysis in the presence of a basic buffer. This hydration reaction is an important partial step in the synthesis sequence from the natural material .alpha.-pinene, via cis-pinane, dihydromyrcene, and dihydromyrcenyl chloride, to the desired fragrance substance, dihydromyrcenol.
It is known, and is described in numerous publications, that one may produce dihyd omyrcenol from .alpha.-pinene using the following multistage synthesis: ##STR1##
.alpha.-pinene is hydrogenated to pinane, with the cis-isomer being much more useful for the subsequent thermal rearrangement whereby the pinane can be selectively converted to dihydromyrcene. The hydrogenation is described in Cocker, W., 1966, J. Chem. Soc. (C), 41-47; and U.S. Pat. Nos. 4,018,842 and 4,310,714.
Cis-pinane may be rearranged thermally in the gas phase at temperatures of 420.degree.-600 .degree. C., whereby dihydromyrcene may be obtained as the main product. However, a whole series of structural isomers is produced: ##STR2##
This thermal rearrangement is described in a number of patents, among them U.S. Pat. Nos. 2,388,084, 2,902,495, and 3,277,206, and Ger. Patents 1,149,000 and 2,744,386. This reaction has been much studied in the scientific literature, beginning some time ago, as illustrated by the following: Ipatieff, V.N., JACS, 75:6222-6225, (I953); Huntsman, W.D., JACS, 80:2252-2254, (1957); Pines, H., JACS, 76:4412-4416, (1954); and Tanaka, J., 1971, Bull. Chem. Soc. Japan, 44:130-132, (1971).
The production of dihydromyrcenol itself is very selective. It is produced in good yield from dihydromyrcenyl chloride and subsequent hydrolysis. Hydrochlorination of dihydromyrcene may be accomplished by introducing dry HCl gas into dihydromyrcene, as described in Brit. Patent 849,567. Various catalysts have been proposed to enhance the reaction. Examples are copper chloride (Jap. Patent 60-058,930) and Lewis and proton acids (Eur. Patent OS 0,170,205). A common feature of all of the hydrochlorination reactions is that one may use the raw product mixture of the thermal rearrangement as the starting material, or one may use pure dihydromyrcene.
In the past, the hydrolysis of the chloride obtained was carried out primarily by addition of NaOH, Ca(OH).sub.2, or CaCO.sub.3 and and Na.sub.2 CO.sub.3. These techniques are described in Brit. Patent 859,567 and U.S. Pat. No. 2,882,323. Recently, the addition of so-called hydroxylation catalysts, e.g. ZnO, has been described in detail (Eur. OS 0,170,205). In the Examples of Eur. OS 0,170,205 it is disclosed that the hydroxylation catalyst ZnO is used in the amount of 0.6 mol per 1 mol dihydromyrcenyl chloride.
The reaction steps from .alpha.-pinene to dihydromyrcenyl chloride, regarded as the State of the Art, comprise a well developed and continually improved technology. It is quite surprising, therefore, that no such highly refined method is available for the hydrolysis step, particularly since this step is the last and crucial step in the production of the desired end product, and the sequence itself is a costly one.
A major disadvantage of the present method of hydrolysis is the long duration of the hydrolysis which duration is taught in the art. For example, after 15 hr reflux dihydromyrcenyl chloride is still present in the amount of 5-10% in the reaction mixture as disclosed in Brit. Patent 859,567. In order to further reduce the residual dihydromyrcenyl chloride it is necessary to extend the hydrolysis time substantially. According to this known method, extremely long reaction times of c. 3-6 da are required to achieve a conversion such that the residual content of dihydromyrcenyl chloride is &lt;0.1%.
Another negative aspect is the costly process which must be employed to separate the desired dihydromyrcenol from the unconverted chloride. In a recent method (Eur. 03 0,170,205) an improvement is described which is realized by the use of so-called hydroxylation reagents, e.g. zinc oxide and magnesium oxide. However, the improvement is only partial, and the hydrolysis times are still very long. In a typical example, after 11 hr under reflux the conversion of the dihydromyrcenyl chloride is only c. 70%.
The small advance represented by the teaching of Eur. OS 0,170,205 over Brit. Patent 859,567, if there is in fact any advance detectable, is nullified by the phase separation problems which occur in processing the product mixture following a successful hydrolysis. These problems are evidently caused by the large amount of zinc oxide used (&gt;/=0.3 mol zinc oxide per 0.5 mol dihydromyrcenyl chloride), which is evidently largely converted to zinc chloride. A need therefore continues to exist for an improved technique of hydrolyzing dihydromyrcenyl chloride to dihydromyrcenol.